1. Field of the Invention
This invention relates to sealing systems and more particularly to cover gas seals for components of liquid metal fast breeder nuclear reactor vessel closure heads.
2. Description of the Prior Art
In nuclear reactors, well known in the art, a vessel houses a core comprised of fuel assemblies about which a coolant is circulated. The vessel is sealed by a closure head. In certain reactors, particularly those identified as liquid metal cooled fast reactors, the closure head comprises one or more rotatable plugs. The rotatable plugs, which may be of varying sizes disposed within one another, provide remote access for refueling as well as sealing of the vessel. Since the plugs must be able to rotate relative to one another to accomplish the refueling purpose, the plugs are typically mounted so as to define an annular clearance among them. The clearance, while allowing rotation of the plugs, also establishes a potential path for release of radioactive fluid mediums, such as gases. Accordingly, seals are provided to prevent release of radioactive materials from the vessel, as well as to prevent fluid communication into the vessel from the surrounding environment. To further prevent communication of the environment, for example including oxygen, with the liquid metal coolant within the vessel, a cover gas is typically provided between the operating level of the reactor coolant and the closure head.
A typical system for sealing the reactor is taught in U.S. Pat. No. 4,078,969 which teaches a liquid metal dip seal fabricated within the rotating plugs, as well as a remotely engageable seal among riser assemblies atop the plugs. The use of dip seals is typical for this application. However, such dip seals typically require extensive auxiliary systems to control the seal fluid level, to fill and drain the seal, to purify the sealing fluid, to establish a desired temperature profile axially through the seal, and to monitor seal temperature and purity. Such support systems can require extensive maintenance as well as requiring complex design. Further, such dip seals have been located away from the reactor coolant pool to prevent contamination of the coolant by the seal fluid, and to permit the dip seal fluid to solidify in whole or in part in order to avoid loss of the dip seal fluid during an undesirable loading such as may occur during a seismic event.
It is thus desirable to provide a sealing arrangement which is simplified relative to existing designs, which provides adequate isolation among the interior and exterior of the vessel, and which further provides this protection in the unlikely event of a seismic occurrence.